1. Field of the Invention
This invention is related to the field of storage management and, more particularly, to software used in storage management.
2. Description of the Related Art
The Network File System (NFS) is a client/server application that lets a computer user view and optionally store and update files on a remote computer as though the files were on the user's own computer. The user's system needs to have an NFS client and the other computer needs an NFS server. NFS was developed by Sun Microsystems and has been designated a file server standard. The NFS protocol provides transparent remote access to shared files across networks. The NFS protocol is designed to be portable across different machines, operating systems, network architectures, and transport protocols. Implementations of NFS exist for a variety of machines, from personal computers to supercomputers.
NFS defines the way in which files are named and where they are placed logically for storage and retrieval. In NFS, a file is placed in a directory (folder in Windows) or subdirectory at the desired place in the tree structure. NFS also specifies conventions for naming files. These conventions may include one or more of, but are not limited to, the maximum number of characters in a name, which characters can be used, and, in some systems, how long the file name suffix can be. NFS also defines a format for specifying the path to a file through the structure of directories.
Using NFS, the user or a system administrator may mount a portion or all of the files available in the NFS file system. The mounted files may be accessed with whatever privileges are associated with the access to each file (e.g. read-only and read-write). The mounted files are logically organized in a file system. A file system is a tree on a single server with a specified root. NFS assumes a file system that is hierarchical, with directories as all but the bottom level of files. Each entry in a file system (file, directory, device, etc.) has a string name. Different operating systems may have restrictions on the depth of the tree or the names used, as well as using different syntax to represent the “pathname,” which is the concatenation of all the “components” (directory and file names) in the name.
A mount point is a position or node in a directory tree on a server at which a file system is mounted. Mount points on a server may be exported to other systems (e.g. servers). When a file system at a mount point on a server is exported to another system, the file system is mounted at a mount point in the importing system.
The NFS protocol uses file handles to uniquely identify files. An NFS server constructs a file handle using the file system identifier (fsid) and the file identifier (fileid) exported by the local file system. The local file system may guarantee that the file system identifier uniquely identifies a file system on that machine, and may guarantee that the file identifier uniquely identifies a file on the specified file system. Thus, the NFS server may guarantee that the file handle uniquely identifies a file on that server. In addition to the file system identifier and file identifier, the file handle may also include export information about the NFS server mount point. NFS supports a lookup procedure for converting file names into file handles.
A MOUNT protocol allows a server to hand out remote access privileges to a restricted set of clients. The mount protocol performs the operating system-specific functions that allow, for example, the attachment of remote directory trees to local file systems. The MOUNT protocol may be used to initiate client access to a server supporting the Network File System (NFS) application. The MOUNT protocol handles local operating system specifics such as path name format and user authentication. Clients desiring access to the NFS program may use the MOUNT protocol to obtain a file handle suitable for use with NFS.
Clustering
Clustering may be defined as the use of multiple computers, for example PCs or UNIX workstations, multiple storage devices, and redundant interconnections, to form what appears to users as a single highly available system. Clustering may be used for load balancing, and parallel processing as well as for high availability. To the outside world, the cluster appears to be a single system. A cluster may be defined as a group of servers and other resources that act like a single system and enable high availability and, in some cases, load balancing and parallel processing.
A common use of clustering is to load balance traffic on high-traffic Web sites. A Web page request is sent to a “manager” server, which then determines which of several identical or similar Web servers to forward the request to for handling. Having a Web farm (as such a configuration is sometimes called) allows traffic to be handled more quickly.
Storage Area Networks
The storage area network (SAN) model places storage on its own dedicated network, removing data storage from the main user network. This dedicated network most commonly uses Fibre Channel technology, a versatile, high-speed transport. The SAN includes one or more hosts that provide a point of interface with LAN users, as well as (in the case of large SANs) one or more fabric switches, SAN hubs and other devices to accommodate a large number of storage devices. The hardware (e.g. fabric switches, hubs, bridges, routers, cables, etc.) that connects workstations and servers to storage devices in a SAN is referred to as a “fabric.” The SAN fabric may enable server-to-storage device connectivity through Fibre Channel switching technology to a wide range of servers and storage devices.
The versatility of the SAN model enables organizations to perform tasks that were previously difficult to implement, such as LAN-free and server-free tape backup, storage leasing, and fill-motion video services. SAN deployment promises numerous advantages, including cost management through storage consolidation, higher availability of data, better performance and seamless management of online and offline data. In addition, the LAN is relieved of the overhead of disk access and tape backup, data availability becomes less server-dependent, and downtime incurred by service and maintenance tasks affects more granular portions of the available storage system.
SAN-Aware File Systems and NFS
SAN-aware file systems may use NFS (Network File System) internally for accessing file system metadata while directing the data traffic directly to the storage over the SAN. An example is the Sanergy file system from IBM Tivoli. This type of clustering solution may be used as a parallel NFS file server only with the capacity to re-export a mount point from any cluster node. This requires the ability to re-export an NFS client mount point from any node in a SAN cluster.
Current NFS implementations do not allow an NFS client (which may be an NFS server) to re-export a file system that it has imported from another NFS server. This limitation is imposed because the file handle defined by the NFS protocol is generally large enough to include information for one server only. For example, a first server may export a file system to be mounted on a second server using the NFS protocol. If the second server then re-exports the same file system to a third server, the file handles obtained from the first server cannot be used by the third server because they might match other file handles on the second server. A prefix added to the first server's file handles by the second server may be used to uniquely identify the file handles, but this generally cannot be done within the constraints of the file handle size limitations. The second server may issue its own file handles that correspond to those from the first server, but this would require a file handle mapping table that would need to persist across reboots of the second server.
Re-exporting a file system may also cause problems for security systems based on server access lists. In the above example, the first server may limit access to the second server, but cannot prevent access from the third server through the second server.